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An Ecosystem Box Model for Estimating the Carrying Capacity of a Macrotidal Shellfish System

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An Ecosystem Box Model for Estimating the Carrying Capacity of a Macrotidal Shellfish System MARINE ECOLOGY PROGRESS SERIES Published December 1 Mar. Ecol. Prog. Ser. An ecosystem box model for estimating the carrying capacity of a macrotidal shellfish system 'IFREMER, Unite de Recherche Ecosystemes Aquacoles, BP 133, F-17390 La Tremblade, France 'IFREMER, Direction de L'Environnement et de llAmenagement Littoral, Laboratoire de Chimie et Modelisation des Cycles Naturels, BP 70, F-29280 Plouzane, France ABSTRACT: An ecosystem model is presented for estimating the shellfish carrying capacity of Marennes-Oleron Bay (France).It incorporates physical and biological processes: horizontal transport of suspended matter, feeding and growth of the cultivated oyster Crassostrea gigas and primary pro- duction. The precision and consistency of the model are tested by comparing simulations to observed data. The model smoothes both spatial and temporal variability of suspended-matter concentrations but reproduces mean levels and seasonal cycles with some accuracy. Carrying capacity of the shellfish system is assessed by computing the sensitivity of oyster growth to oyster abundance. Model results clearly inbcate a density dependence of oyster growth. When stock is adjusted from 20% to 200% of the present value, maximal dry weight of oysters shows a mean decrease of approximately 25%. The spatial heterogeneity of the growth response is a consequence of the low level of primary production within the shellfish area. The hydrodynan~icregime of the bay strongly controls the carrying capacity of the shellfish system: flushing time and available light energy are found to be 2 determining factors which prevent the phytoplankton from thriving in the bay; tidal currents ensure a fast renewal of food. Nevertheless, phytoplanktonic production accounts for a non-negligible part of food filtered by oysters and is identified as an Important food source when stock level is low. The validity of the model is h- ited mainly by the present description of the physical transport of suspended and deposited matter. KEY WORDS: Oyster farming . Carrying capacity . Ecological box model . Crassostrea gigas. Stock sensitivity INTRODUCTION This approach has the advantage of being simple and reliable. However, it provides a very restrictive picture Carrying capacity is a fundamental concept in shell- of the carrying capacity since it does not account for fish culture. Following Incze et al. (1981) and Rosen- potential variations of this capacity in space and time. berg & Loo (1983), it corresponds to the ability of the In addition, there are empirical models which depict system to support shellfish production. Mathematical some aspects of the interrelationship between shellfish tools are generally used to estimate carrying capacity. and food supply. The model developed by Incze et al. The complexity of the tools depends on the scientific (1981) estimates the optimal size of a mussel culture requirements, the ecosystem considered and the avail- from both seston concentration and water fluxes enter- ability of data. ing the system. In the model constructed by Wildish & Based on historical data, global models allow the Kristmanson (1979), the growth potential of suspen- overall production of a system to be represented as an sion-feeding macrobenthos is determined by the empirical function of the biomass (HBral et al. 1986). supply rate of ATP associated with advected seston. Smaal et al. (1986) correlate current velocity and shell- fish biomass with seston depletion for different values 'Present address: CETIIS. 24 boulevard Paul Vaillant-Cou- of mussel bed length. In order to estimate the carrying turier, F-94200 Ivry-sur-Seine, France capacity of their system, Carvet & Mallet (1990) divide 0 Inter-Research 1994 Resale of full article notpermltted 118 Mar Ecol. Prog. Ser 115: 117-130, 1994 food supply, determined from measurements of tidal temporal framework. The goal is to establish relations exchanges and food concentration, by food demand, between oyster growth and biomass and also to point estimated from in situ measurements of grazing. out the physical and biological processes which deter- Whatever their respective efficiency, the validity of mine the carrying capacity of the bay. these models is limited to restricted spatial and tempo- The study is composed of 2 parts. First, the consis- ral scales, as they model neither the impact of shellfish tency and precision of the model are assessed by com- culture on the overall dynamics of the system, nor the panng calculations to the data. Second, the model is regeneration of food within the shellfish system, nor used to quantlfy the sensitivity of oyster growth to oys- the spatial variability of both biological demand ter biomass. and physical characteristics of the system. When com- plex ecosystems are considered, a more explanatory, process-based model is needed (Heral et al. 1983a). MATERIAL AND METHODS The purpose of the present work is to refine the con- cept of carrying capacity by developing an ecosystem General description of the bay. Marennes-Oleron model which describes the physical and biological Bay is situated in southwest France. It is delimited by dynamics of a shellfish system. Oleron Island on its western side and by the continen- Macrotidal estuaries are favourable areas for shell- tal coast on its eastern side (Fig. 1). Water exchange fish culture. Being protected from the turbulence of the with the sea occurs in the north through the narrows of sea, such estuaries offer good trophic conditions due Antioche and in the south through the narrows of Mau- to strong tidal currents which ensure an intensive musson. The area of the bay is 200 km2 and its mean renewal of food within the area. Nevertheless, the depth is about 5 m, with an average of 10 m in the potential production of these areas is not unlimited. The case of Marennes-Oleron Bay, France, is a good illustration of this fact. Since its introduction into the bay in 1972, Japanese oyster Crassostrea gigas has shown a progressive reduction in its capacity for growth. The period of growth required for the oyster to reach a commercially valuable size has increased from 1.5 to 4 yr during the period 1972 to 1985 (Heral et al. 1986).At the same time, the mortality rate of the oyster has also increased. As shown by Heral et al. (1988), such physiological problems in C. gigas may have a trophic origin. By increasing the cultivated biomass, oyster-farmers may have reduced the food supply to the oysters. Because of the economic consequences of this situa- tion, the problem of estimating the carrying capacity of Marennes-Oleron Bay has become a priority. Bacher (1989) has developed a first model for this which cou- ples the feeding behaviour of Crassostrea gigas and the physical transport of food. It reveals density depen- dence of oyster growth and strong intra-specific com- petition. However, the model is based on the assump- tion that renewal of food by primary production is negl~giblewhen compared to the inputs by tidal cur- rents at the ocean boundary. On this basis, the 'stock/ growth' relationship computed by Bacher's model infers that sensitivity of the carrying capacity to pri- mary production does not depend on stock level. This assumption is reasonable for small changes in stock level but may become unreasonable when large varia- tions are considered. The present model is designed to incorporate feed- ing behaviour and growth of oysters, primary produc- Fig. 1 Marennes-Oleron Bay, France, showing the maln cir- tion and physical transport in the same spatial and culation features. Isobaths in meters Raillard & Menesguen: Box model of a shellfish ecosystem 119 north. The bay is a dynamic environment with a tidal (L2). Elementary cells of the hydrodynamic model range up to 5 m. The local hydrodynamic regime is were aggregated into boxes in order to match as far as regulated mainly by tidal currents, resulting in a possible the physical and biological gradients in the southward residual transport of the water masses. Res- bay including those of: current velocity and direction, idence time of a water parcel in the bay is between salinity (due to Charente Estuary), and oyster distribu- 5 and 10 d, depending on tide and meteorological con- tion (flooding areas/gullies). The amplitude of the tidal ditions. The River Charente is the major source of excursion, which is responsible for water mixing, freshwater input into the bay. Although river flow is determines the mean geographic extent of the boxes. negligible (1 %) in comparison to oceanic inflow (99 X), The segmentation of Marennes-Oleron Bay is shown in it can be an important source of nutrients, thus increas- Fig. 2. For box volume computation, only marine area ing primary production (Ravail et al. 1988). During is taken into account. periods of flooding, the waters of Gironde enter the When interaction with biological processes is absent, bay (Dechambenoy et al. 1977) and have been shown the concentration of seston Ci in Box i over time t is to increase nutrient levels significantly. calculated by the following equation: Crassostrea gigas is cultivated in the bay. Its esti- mated biomass is approximately l l0 000 t and annual biological production can rise up to 35 000 t (A. Bodoy pers, comm.). This latter value marks Marennes- where n is the total number of boxes, V, is the volume Oleron Bay as one of the most productive areas for oys- of Box i, A, is the advective flow coming out of Box i, ters in the northeast Atlantic. Ecosystem model. Physical sub-model: A hydrody- namic model was developed (Anon. 1979) to calculate vertically averaged current velocity and direction throughout the bay. Considering the vigorous hydro- dynamic regime and the shallow bathymetry of the bay, it can be assumed that the water column 1s verti- cally homogeneous. In this model, tidal forcing is the only factor determining water circulation.
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